Thanks to effective vaccination, polio is nearly eradicated and only a few hundred people are stricken worldwide each year.

But researchers in PNAS have reported alarming findings: a mutated virus was able to resist the vaccine protection to a considerable extent in the Congo in 2010. The pathogen could also potentially have infected many people in Germany.

The polio epidemic in the Congo in 2010 was especially serious. 445 people were verifiably infected, mostly young adults. The disease was fatal for 209 of them. This high mortality rate is surprising. Also important was the fact that many of those affected had apparently been vaccinated: Surveys indicated that half of the patients remembered having received the prescribed three vaccination dosages. To date the vaccination has been considered a highly effective weapon for containing the polioviruses that cause the disease.

"We isolated polio-viruses from the deceased and examined the viruses more closely." explains Dr. Jan Felix Drexler, who carried out the study at the Institute for Virology of the University Hospital of Bonn under the supervision of Prof. Christian Drosten, along with colleagues from Gabon, Dr. Gilda Grard and Dr. Eric Leroy. "The pathogen carries a mutation that changes its form at a decisive point."

The result: the antibodies induced by the vaccination can hardly block the mutated virus and render it harmless.

The researchers have examined the success with which the new pathogen evades the immune system. To this purpose, they tested, among others, blood samples from 34 medical students of the University of Bonn. All of them were vaccinated in childhood with the usual methods against polio. And very successfully, as an initial test showed: The antibodies in the blood of the test subjects had no problem combating "normal" polio viruses.

The situation was different with the mutated virus; the immune reaction was much weaker here. "We estimate that one in five of our Bonn test subjects could have been infected by the new polio virus, perhaps even one in three," says Drosten.

Eradication possible

The World Health Organization (WHO) has undertaken the goal of eradicating the polio virus in coming years. The role model here is smallpox – thanks to a consistent vaccination strategy, the earth has been classified as free of smallpox since 1980. The chances are principally good that something similar could succeed again: The polio virus can also only be transmitted from person to person.

There are thus no pathogen reservoirs in animals from which the disease could spread repeatedly. Similar to with smallpox, the polio vaccines also offer extraordinary protection. This, however, does not apply when the virus mutates. "When such an altered pathogen encounters a population that has not been consistently vaccinated enough, then things get dangerous," the scientists warn.

The polio epidemic in the Congo was stopped with a massive vaccination program and hygiene measures. Even the current vaccines thus appear to be good enough to be effective when they are promptly and consistently administered.

The new pathogen is nonetheless a warning: "We can't afford to sit back and do nothing," the scientists warn. "We need to further increase the vaccination rate and develop new, more potent vaccines. Only in this way do we have a chance of permanently vanquishing polio."